Oil tempered steel wire excellent in coiling workability and method for production thereof

ABSTRACT

An oil tempered wire, characterized in that it comprises a wire material and, formed on the surface thereof, a coating film comprising one or more inorganic salts selected from the group consisting of a sulfate, a silicate and a borate. The coating film optionally further comprises fats and oils and/or a soap exhibiting lubricity. The oil tempered wire has a coating film exhibiting satisfactory heat resistance and good lubricity and thus can secure good coiling workability even in the case of a high strength wire or a wire having a heteromorphic section.

FIELD OF THE INVENTION

The invention relates to an oil tempered wire to be used as a material of valve springs of internal-combustion engines and a material of bridging springs. Furthermore, the invention relates to a production method for the oil tempered wire which enables lubricity between a material and a tool to be improved remarkably in spring working so that the productivity and precision of the springs can be improved.

BACKGROUND ART

With respect to a wire material to be used as a material for the above-mentioned various kinds of the spring, it is common to produce the wire by heat treatment of quenching and tempering, so-called oil tempering, to improve the properties, after wire-drawing process. The wire material produced in such a manner is to be formed into a spring by coiling and in such process, in terms of improvement of the productivity, good lubricity between the wire material and the spring forming tool is very important.

Various techniques to improve the lubricity in the case of coiling oil tempered wires have been proposed so far and the following (1) to (3) are representative examples: (1) a method of controlling components, the thickness and the adhesion property of an oxide scale formed on the surface of an oil tempered wire to be proper values to produce an oxide coating with good workability (e.g. Japanese Patent Application Publication No. 5-14771); (2) a method of applying a lubricating oil having an anti-rust property to an oil tempered wire in place of a rust-preventing oil; and (3) a method of controlling the thickness of an oxide scale formed on the surface of an oil tempered wire to be a prescribed thickness or thinner and forming a coating film of an alkyd resin, a resin mainly containing an amino acid, or the like as means for assisting the coiling property (e.g. Japanese Patent No. 3,003,831).

On the other hand, it has been said in recent years that an oil tempered wire to be used for valve springs and the like of automotives are required to satisfy the following requirements: (1) to meet the lightweight requirement for automobiles, a wire is required to have a further improved strength and a thinner diameter and tends to be used under high design stress: and (2) a wire with a modified cross-section, for example an oval shape, tends to be used more increasingly to install a spring in a narrower space and at the same time assure a needed stroke. In a coiling work of the wire with a modified cross-section, to secure needed spring precision, it is required to strictly suppress rotation of the wiring material during the coiling.

Particularly, in recent years, in conformity to the requirements for high performance and lightweight, designs of spring forms have to be proper to be used under high load stress and materials with higher strength and hardness tend to be used often for producing springs with a large amount of plastic deformation degree in spring machining.

A spring is required to have satisfactorily small diameters and wide spring pitches so as to have a ratio (D/d) of an average diameter of spring (D) to the wire diameter (d) smaller than 3.5 as a practical evaluation standard.

At the time of coiling for obtaining such a spring, because of wear of a material against a tool and heat generation by working, the temperature in the interface of the material and the tool is increased. At the time of coiling, although good workability is assured by an oxide coating formed on the wire material surface and application of a rust preventing oil, the heat resistance of the lubricating coating cannot necessarily be sufficient in such a lubricating method to result in occurrence of adhesive wear between the wire material and the tool, and difficulty of efficient production of a spring with a prescribed shape.

Also, in the case of producing a spring from a wire material with a modified cross section, in order to keep the precision high, it is required to carry out coiling the material by fixing the material by several guides and pins with small clearance. Springs produced from high strength wire materials with modified cross-sections tend to be used increasingly.

Under the above-mentioned situation, it possibly leads to the following undesirable consequences that only a conventional oxide coating on the oil tempered wire surface and a rust preventing oil are used because the lubricating effect of the rust preventing oil is not so significant: 1: owing to insufficient lubricity between a coiling tool such as a coiling pin and a guide and the wire surface, seizure easily occurs to shorten the tool life and therefore, the operation of the machine has to be stopped and the tool has to be replaced or repaired to result in decrease of productivity: 2: in the case the seizure occurs, dispersion of the diameter and the free length (the length of the spring without any load) of a product spring tends to be wide and the precision of the product is worsened and the yield of the product tends to be decreased as well: and 3: further, the seizure as described causes harmful linear scratches on the spring surface and causes adverse effects on the spring life in some cases.

In view of the above-mentioned state of the art, it is an object of the present invention to provide a lubricating coating with sufficient heat resistance and accordingly secure good lubricity and to provide an oil tempered wire having secured good coiling workability even if the wire is a high strength wire or an oil tempered wire with a modified cross-section.

DISCLOSURE OF THE INVENTION

An oil tempered wire of the present invention achieving the above-mentioned object is characterized in that a coating film layer comprising one or more inorganic salts selected from a group consisting of a sulfate, a silicate and a borate is formed on a surface of a wire material.

As the inorganic salt of the coating film, one or more compounds selected from a group consisting of sodium silicate, potassium silicate, magnesium silicate, sodium sulfate, potassium sulfate, sodium borate and potassium borate can be used.

With respect to the oil tempered wire of the invention, the coating film layer comprising the inorganic salt is preferable to further contain a fat and oil and/or a soap having lubricity if necessary. As the fat and oil to be added to the coating film, waxes such as petroleum waxes can be used, and as the soap, one or more salts selected from sodium salts or potassium salts of stearic acid, palmitic acid, oleic acid, linoleic acid and linolenic acid can be used. In the case of adding a fat and oil and/or a soap, the content of them in the coating film is preferably adjusted to be 5 to 50% by mass.

In the oil tempered wire of the invention, it is also preferable to further apply a rust preventing oil on the coating film surface if necessary and accordingly the anti-rust property of the oil tempered wire can further be improved.

On the other hand, to produce the oil tempered wire of the invention, the lubricant coating film may be formed on the wire material surface by immersing an oil tempered wire in an aqueous solution containing one or more inorganic salts selected from a group consisting of a sulfate, a silicate and a borate or in an aqueous solution containing a fat and oil and/or a soap having lubricity in addition to the inorganic salts, if necessary, for forming a coating of the aqueous solution on the wire material surface, and then drying and removing water from the coating of the aqueous solution. To carry out the method, the solid matter concentration in the aqueous solution coating is preferably 1.5 to 30% by mass.

BEST MODE FOR CARRYING OUT OF THE INVENTION

Inventors of the present invention have made various investigations and experiments of means of preventing seizure from a viewpoint of improving lubricity and heat resistance in coiling of an oil tempered wire for a spring, have found the above-mentioned object could be accomplished in the above-mentioned manner, and have completed the present invention.

The lubrication mechanism of a wire and a tool in presently carried out spring machining is attributed to an oxide film on the wire surface formed during the production process of the oil tempered wire and the lubricity of a rust preventing oil applied to the surface of a wire product presently made available. On the other hand, only improvement of the lubricity by the rust preventing oil alone is insufficient in order to satisfy the requirement for an oil tempered wire with high strength and a modified cross-section, requirement for a product spring with a high precision, requirement for improvement of the productivity by high speed coiling.

Also, with respect to the oxide coating film on the wire surface, it is required for the oxide coating film to maintain the rust preventing oil and have a certain coating thickness to moderate the direct contact between the wire material and a tool, however if the thickness becomes so thick, the adhesion property of the oxide coating film is deteriorated to result in undesirable consequence that the coating is separated before a coiling pin. Such an undesirable consequence leads to deterioration of the coiling workability.

Accordingly, the oxide coating film has to be controlled to have a proper thickness and a method of controlling the thickness of an oxide coating film is proposed (e.g. in Japanese Patent Publication No. 5-14771), but it is still insufficient to satisfactorily meet the requirement of lubricity improvement, which has become more and more demanded, in coiling machining.

To satisfy such a requirement, the inventors of the invention have divided the functions which the oxide coating film and the rust preventing oil in the coiling machining should carry out as follows and have noted that the composition of a coating film to be formed on the spring surface should properly be controlled so as to execute the functions: (a) a function of moderating the direct metal contact of a tool such as a pin with the material (the wire surface); (b) a function of decreasing the friction coefficient of the rust preventing oil as a lubricating oil; and (c) a function as a rust preventing oil for the wire material.

The above-mentioned function (a), it has been found that not only the oxide coating film on the wire surface formed in the oil tempering treatment but also deposition and retention of inorganic fine hard particulates on the wire material surface can positively reinforce the role as a buffering agent having heat resistance between the tool and the material at the time of machining.

Also, it has been found that a sulfate, a silicate and a borate are optimum as practical inorganic fine particulates. That is, if a coating film layer containing the above-mentioned various inorganic salts is formed on the oil tempered wire material surface, the above-mentioned function (a) can efficiently be exhibited. Further, as the means for reinforcing the above-mentioned function (b), a lubricating component such as a fat and oil and a soap is positively added to the coating film layer, based on necessity, to further improve the lubricity. With respect to the above-mentioned function (c), a rust preventing oil is applied further to the surface of the coating film layer, based on necessity, to exhibit the function. Additionally, it has been confirmed that the respective components in the coating film layer do not cause any adverse effect on the anti-rust property of the rust preventing oil.

The oil tempered wire of the invention is basically the wire wherein the coating film containing the inorganic salt is formed on the surface thereof and as the inorganic salt to be used in this case, a sulfate such as potassium sulfate and sodium sulfate; a silicate such as sodium silicate, potassium silicate, and magnesium silicate; and a borate such as sodium borate and potassium borate can be exemplified and one of them can be used, or two or more of them can be used in combination. Among them, in terms of the heat resistance and environmental affinity, particularly preferable inorganic salts are a silicate such as sodium silicate, potassium silicate, and magnesium silicate and a sulfate such as sodium sulfate and potassium sulfate.

Besides the above-mentioned inorganic salt, the coating film layer to be formed on the wire material surface of the invention may contain a fat and oil and/or a soap if necessary and these components have a function of providing lubricity to the coating film [the above-mentioned function (b)].

As the above-mentioned fat and oil, waxes such as petroleum waxes can be exemplified and as the above-mentioned soap, one or more salts selected from sodium salts or potassium salts of stearic acid, palmitic acid, oleic acid, linoleic acid and linolenic acid can be used. The content of the fat and oil and/or the soap in the coating film is preferably in a range of 5 to 50% by mass. That is, if the content of the fat and oil and/or the soap in the coating film is less than 5% by mass, the effect to provide the lubricity is insufficient and if it exceeds 50% by mass, the ratio of the inorganic salt is lowered to lead to insufficiency of the strength and heat resistance of the coating film and difficulty to withstand strong machining.

Formation of the lubricant coating film layer containing the above-mentioned respective components on the oil tempered wire provides sufficiently high heat resistance to the lubricant coating film and secures the good lubricity and accordingly gives an oil tempered wire having good coiling workability even in the case of an oil tempered wire having high strength or a modified cross-section. In such an oil tempered wire is also preferable to be further coated with a rust preventing oil on the coating film surface, based on the necessity. Accordingly, the oil tempered wire is provided with the anti-rust property [the above-mentioned function (c)]. As the rust preventing oil to be used for this case, those which have been used conventionally may be used and mixtures of petroleum type hydrocarbons and rust preventing additives can be exemplified.

The oil tempered wire of the invention is the wire wherein the coating film layer containing the above-mentioned components is formed on the surface of the wire material and with respect to a method of forming the coating film layer, the lubricant coating film can be formed on the wire material surface, for example, by immersing an oil tempered wire in an aqueous solution containing the various inorganic salts or an aqueous solution further containing, if necessary, a fat and oil and/or a soap having lubricity at proper ratio other than the inorganic salts for forming an aqueous solution coating on the wire material surface, and then drying out the water in the aqueous solution coating.

In the case of carrying out the method, the solid matter concentration (that is the concentration of the inorganic salts and if necessary, the fat and oil and/or the soap) in the aqueous solution is preferably 1.5 to 30% by mass. If the solid matter concentration is lower than 1.5% by mass, the effect of the lubricant coating film cannot be obtained and if it exceeds 30% by mass, these components are oversaturated and unevenly distributed in the coating film to result in a problem of uniformity of the coating film.

The invention is effective in the case of application to a spring having a spring diameter wherein a ratio (D/d) of the average diameter (D) and the wire diameter (d) of the spring is lower than 3.5 and having a large spring pitch; however the invention should not be limited to such a spring and exerts the objective effects also in application to springs having a ratio (D/d) of 3.5 or higher.

EXAMPLES

Hereinafter, the features and advantages of the present invention will be described more in detail along with Examples. However, it is not intended that the invention be limited to the Examples to be illustrated. Modifications and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the invention.

Examples

High strength oil tempered wires for valve springs which were made of various steels shown in Table 1 were produced by conventional quenching and tempering treatment. Oxide coating films with the same thickness as that of a conventional agent were found formed on the surfaces of the respective oil tempered wires (reference to Table 2). In this case, wires having oval cross-sectional shapes and wires having round shapes were produced and the tensile strength of them was measured. TABLE 1 Chemical component composition (% by mass) Steel type C Si Mn P S Cu Ni Cr V Remarks A 0.54 1.43 0.69 0.013 0.008 0.01 — 0.71 — Silicon—chromium steel B 0.63 1.44 0.62 0.011 0.010 0.01 — 0.67 0.092 High strength silicon—chromium steel C 0.59 2.00 0.89 0.012 0.009 0.01 0.26 0.99 0.095 High strength silicon—chromium steel

Lubricant coating films were formed on the surfaces of the above-mentioned respective oil tempered wires by the following methods 1 to 5. In this case, to reinforce the anti-rust property, as rust preventing oils for ferrous materials, commonly used mixtures of petroleum type hydrocarbons and rust preventing additives were used for applying to the coating films.

[Coating Film Formation Method 1]

The coating film was formed on the wire material surface by immersing an oil tempered wire in an aqueous solution at 70° C. containing sodium silicate, potassium silicate, and potassium sulfate and adjusted to have a solid matter concentration 11% by mass and removing water from the formed solution coating by drying.

[Coating Film Formation Method 2]

To an aqueous solution containing sodium silicate, potassium silicate, and potassium sulfate was added a wax. The coating film was formed on the wire material surface by immersing an oil tempered wire in the aqueous solution at a room temperature (25° C.) with a solid matter concentration 11.5% by mass and removing water from the formed solution coating by drying.

[Coating Film Formation Method 3]

To an aqueous solution containing sodium silicate and potassium silicate was added a wax. The coating film was formed on the wire material surface by immersing an oil tempered wire in the aqueous solution at a room temperature (25° C.) with a solid matter concentration 8.5% by mass and removing water from the formed solution coating by drying.

[Coating Film Formation Method 4]

To an aqueous solution containing sodium silicate and potassium silicate was added sodium stearate as a soap component. The coating film was formed on the wire material surface by immersing an oil tempered wire in the aqueous solution at a room temperature (25° C.) with a solid matter concentration 10% by mass and removing water from the formed solution coating by drying.

[Coating Film Formation Method 5]

To an aqueous solution containing sodium silicate, potassium silicate, and magnesium silicate was added a wax. The coating film was formed on the wire material surface by immersing an oil tempered wire in the aqueous solution at a room temperature (25° C.) with a solid matter concentration 8.5% by mass and removing water from the formed solution coating by drying.

The obtained oil tempered wires were subjected to tensile strength measurement and at the same time coiled to produce sprigs having the various D/d ratios, and the dispersion σ(D) of the coil diameters and the dispersion σ(H) of the free lengths of the springs were investigated. The dispersion σ(H) of the lengths was shown as the dispersion per 1 mm free length [dispersion σ(H)/H] calculated by dividing the dispersion σ(H) by the free length H (mm). Also, an investigation was carried out in which the number of coils is counted until a tool is needed to be grinded or repaired due to occurrence of a linear scratch (galling) caused by poor lubricity between the wire surface and a tool during the spring machining, observing the spring surfaces appropriately.

The results are collectively shown in the following Table 2. Table 2 also show the results of the evaluations carried out similarly to wire materials coated with common rust preventing oils alone (Nos. 1, 3, 6, and 11 in Table 2). TABLE 2 Dispersion Dispersion The number of Coating film Cross-section Tensile Oxide film of of coils until formation Steel shape of wire Wire diameter strength thickness diameter free length Occurrence re-grinding of tool No. method type material (mm) (MPa) (μm) D/d σ(D) σ(H)/H of scratches (pieces) 1 — A Oval Major axis: 4.0 1920 1.6 3.0 0.042 0.0036 Found 2000 2 1 A Oval Major axis: 4.0 1920 1.6 3.0 0.026 0.0022 None 10000 3 — B Oval Major axis: 4.4 2045 1.8 3.8 0.035 0.0022 Found 2000 4 1 B Oval Major axis: 4.4 2045 1.8 3.8 0.023 0.0017 None 10000 5 3 B Oval Major axis: 4.4 2045 1.8 3.8 0.021 0.0015 None 10000 6 — C Round 5.0 2060 2.1 3.2 0.045 0.0049 Found 1500 7 1 C Round 5.0 2060 2.1 3.2 0.035 0.0023 None 10000 8 2 C Round 5.0 2060 2.1 3.2 0.038 0.0022 None 10000 9 3 C Round 5.0 2060 2.1 3.2 0.032 0.0022 None 10000 10 4 C Round 5.0 2060 2.1 3.2 0.039 0.0026 None 10000 11 — C Round 3.2 2150 2.0 2.9 0.129 0.0096 Found 1000 12 3 C Round 3.2 2150 2.0 2.9 0.043 0.0028 None 10000 13 2 C Round 2.7 2200 1.8 7.6 0.022 0.0026 None 10000 14 3 C Round 6.1 2040 2.0 3.9 0.031 0.0012 None 10000 15 5 C Round 6.1 2040 2.0 3.9 0.027 0.0013 None 10000

According to the results, the following can be understood. At first, as compared with those coated conventionally only with the oxide coating film and the rust preventing oil (Nos. 1, 3, 6, and 11), the coils (Nos. 2, 4, 5, 7 to 10, and 12 to 15) satisfying the factors defined by the invention had narrow coil diameter dispersion and free length dispersion and the number of coiling times until the time of occurrence of scratches was high and no pin grinding was needed until completion of the coiling of 1 lot (10,000 coils). In other words, it was found that the oil tempered wires of the invention gave springs as products excellent in the precision at a high productivity, scarcely damaging the tool, which requires wasteful preparatory time. According to the investigations carried out by inventors of the invention, it was also confirmed that the lubricating components to be employed in the invention did not cause any adverse effects in the quality and working environments in tempering treatment and shot-pinning in the spring manufacturing process.

INDUSTRIAL APPLICABILITY

The invention is constituted as described above and in the coiling of a spring, the heat resistance of a lubricating film between a wire and a tool is improved and the lubricity itself is also improved, so that in the case of manufacturing a coil with a small D/d ratio from a high strength wire by severe machining, the machining can be carried out easily and the invention practically causes the following effects (a) to (c). (a) The dispersion of the free lengths of springs as products and the dispersion of coil diameters of the springs are narrowed to improve the machining yield. (b) The lives of machining tools are prolonged and the time taken to replace or repair the tools is shortened to improve the productivity. (c) The scratches by galling of the surfaces of springs are hardly formed to improve the reliability of the products.

Further, the invention providing the above-mentioned oil tempered wire is (1) optimum in the case of machining a wire with modified cross-sections, e.g. an oval cross-sectional shape, requiring to have better lubricity; (2) suitable in the case of coiling a wire with large diameter and having high plane pressure to tool surfaces; and (3) applicable to coiling at an increased coiling speed owing to the high heat resistance of the coating film and thus contributes to improvement of the productivity. 

1. An oil tempered wire excellent in coiling workability, wherein a coating film comprising one or more inorganic salts selected from a group consisting of a sulfate, a silicate and a borate is formed on a surface of a wire material
 2. The oil tempered wire according to claim 1, wherein the inorganic salt is one or more inorganic salts selected from a group consisting of sodium silicate, potassium silicate, magnesium silicate, sodium sulfate, potassium sulfate, sodium borate and potassium borate.
 3. The oil tempered wire according to claim 1, wherein the coating film layer comprising the inorganic salt further contains a fat and oil and/or a soap having lubricity.
 4. The oil tempered wire according to claim 3, wherein the fat and oil is a wax, and the soap is one or more salts selected from sodium salts or potassium salts of stearic acid, palmitic acid, oleic acid, linoleic acid and linolenic acid.
 5. The oil tempered wire according to claim 3, wherein the coating film contains the fat and oil and/or the soap in a content of 5 to 50% by mass.
 6. The oil tempered wire according to claim 1, wherein a rust preventing oil is further applied to the surface of the coating film.
 7. A production method of the oil tempered wire excellent in coiling workability according to claim 1, comprising immersing an oil tempered wire in an aqueous solution containing one or more inorganic salts selected from a group consisting of a sulfate, a silicate and a borate for forming a coating of the aqueous solution on the wire material surface, and then drying and removing water from the coating of the aqueous solution.
 8. A production method of the oil tempered wire excellent in coiling workability according to claim 2, comprising immersing an oil tempered wire in an aqueous solution containing one or more inorganic salts selected from a group consisting of a sulfate, a silicate and a borate as well as a fat and oil and/or a soap having lubricity for forming a coating of the aqueous solution on the wire material surface, and then drying and removing water from the coating of the aqueous solution.
 9. The oil tempered wire according to claim 2, wherein the coating film layer comprising the inorganic salt further contains a fat and oil and/or a soap having lubricity.
 10. The oil tempered wire according to claim 4, wherein the coating film contains the fat and oil and/or the soap in a content of 5 to 50% by mass.
 11. The oil tempered wire according to claim 2, wherein a rust preventing oil is further applied to the surface of the coating film.
 12. The oil tempered wire according to claim 3, wherein a rust preventing oil is further applied to the surface of the coating film.
 13. The oil tempered wire according to claim 4, wherein a rust preventing oil is further applied to the surface of the coating film.
 14. The oil tempered wire according to claim 5, wherein a rust preventing oil is further applied to the surface of the coating film.
 15. A production method of the oil tempered wire excellent in coiling workability according to claim 3, comprising immersing an oil tempered wire in an aqueous solution containing one or more inorganic salts selected from a group consisting of a sulfate, a silicate and a borate as well as a fat and oil and/or a soap having lubricity for forming a coating of the aqueous solution on the wire material surface, and then drying and removing water from the coating of the aqueous solution.
 16. A production method of the oil tempered wire excellent in coiling workability according to claim 4, comprising immersing an oil tempered wire in an aqueous solution containing one or more inorganic salts selected from a group consisting of a sulfate, a silicate and a borate as well as a fat and oil and/or a soap having lubricity for forming a coating of the aqueous solution on the wire material surface, and then drying and removing water from the coating of the aqueous solution.
 17. A production method of the oil tempered wire excellent in coiling workability according to claim 5, comprising immersing an oil tempered wire in an aqueous solution containing one or more inorganic salts selected from a group consisting of a sulfate, a silicate and a borate as well as a fat and oil and/or a soap having lubricity for forming a coating of the aqueous solution on the wire material surface, and then drying and removing water from the coating of the aqueous solution. 